The present invention relates to an in-cylinder injection type internal combustion engine.
Recently, an in-cylinder injection type internal combustion engine for improving an engine output and a fuel economy has been practiced. In the internal combustion engine of this kind, as disclosed in Unexamined Published Japanese Patent Application 8-312404, for example, fuel is injected in a suction stroke while making an air/fuel ratio feedback control performing the stoichiometric air/fuel ratio as a target value in a medium/high load region that demands an engine output. In a low load region, on the other hand, the fuel is injected in the compression stroke while making an open loop control at a super-lean air/fuel ratio (e.g., 25 or higher) which is far leaner than the stoichiometric air/fuel ratio.
In other words, the compression stroke injection in the in-cylinder injection type internal combustion engine effects the laminar combustion at the super-lean air/fuel ratio aiming mainly at improving the fuel economy. As a result, the compression stroke injection of the in-cylinder injection type internal combustion engine is executed exclusively in the lean air/fuel ratio region.
However, the internal combustion engine of this kind is troubled by various problems: that the drive is performed at a far leaner air/fuel ratio than that of the existing internal combustion engine for injecting the fuel into the intake passage so that the exhaust gas temperature drops to inactivate the once active catalyst; and that the fuel injection timing varies between the suction stroke and the compression stroke in dependence upon the drive state so that the torque fluctuates.
We have made experiments by changing the target air/fuel ratio of the compression stroke injection to the outside the super-lean air/fuel ratio region so as to find effective means for solving those problems, and have found that the engine characteristics at the time when the compression stroke injection was effected in the air/fuel ratio region in the vicinity of the stoichiometric air/fuel ratio were drastically different from those at the suction stroke injection time.
An object of the invention is to provide an in-cylinder internal combustion engine which is enabled to solve the problems inherent to the in-cylinder internal combustion engine by exploiting the engine characteristic changes at the time when the air/fuel ratio region of the compression stroke injection is extended.
According to a first aspect of the invention, there is provided an in-cylinder injection type internal combustion engine which has: a first operation mode for performing a suction stroke injection while making an air/fuel ratio feedback control performing a stoichiometric air/fuel ratio as a first target air/fuel ratio; and a second operation mode for performing a compression stroke injection while making an air/fuel ratio feedback control performing a predetermined air/fuel ratio in the vicinity of the stoichiometric air/fuel ratio as a second target air/fuel ratio, and selectively switching each drive mode under a predetermined condition.
According to this in-cylinder injection type internal combustion engine, the suction stroke injection at the stoichiometric air/fuel ratio or the compression stroke injection in the vicinity of the stoichiometric air/fuel ratio is selectively performed so that the exhaust gas characteristics or the output characteristics are improved by exploiting the changes of the engine characteristics accompanied with the switching of the injection modes.
In the embodiment of the invention, it is preferable that the operation mode switching means switches the operation mode to the first operation mode when the internal combustion engine is in the medium/high load drive state and switches the operation mode to the second operation mode when the internal combustion engine is in the drive state demanding a temperature rise of the catalyst of an exhaust purifying catalyst. It is preferable that the second target air/fuel ratio in the air/fuel ratio feedback control in the second operation mode is set at a slightly leaner air/fuel ratio than the stoichiometric air/fuel ratio.
According to our aforementioned experiments, the emissions of the reduced components such as carbon monoxide (CO) or the excess oxygen (O2) at the time of performing the compression stroke injection in the vicinity of the stoichiometric air/fuel ratio are more than those at the suction stroke injection at the stoichiometric air/fuel ratio. With the compression stroke injection in the vicinity of stoichiometric air/fuel ratio, more specifically, a rich air/fuel ratio region is established locally in the combustion chamber so that the incomplete combustion occurs to produce much CO while leaving much O2 in the remaining combustion chamber regions. As a result, the emissions of CO and O2 at the compression stroke injection time in the vicinity of the stoichiometric air/fuel ratio are more than those at the suction stroke injection time at the stoichiometric air/fuel ratio. When the CO and O2 exhausted from the combustion chamber reach the catalyst, moreover, the CO and O2 cause oxidations under the action of the catalyst so that the catalyst temperature is raised by the result heat of reaction. In the medium load drive region, therefore, the necessary engine output can be generated, and the catalyst temperature can be raised in response to its demand. By the compression stroke injection at the lean air/fuel ratio, the rich misfire or the poor fuel economy is prevented, and the emission of smoke is suppressed to improve the exhaust characteristics.
In the embodiment of the invention, air/fuel ratio detecting means is preferably constructed of oxygen concentration detecting means. By using this oxygen concentration detecting means, the sensor system relating to the air/fuel ratio feedback control is made simple and inexpensive, and the control itself is simplified.
On the other hand, the output of the oxygen concentration detecting means is ordinarily turned at the stoichiometric air/fuel ratio between a first output value and a second output value. In the air/fuel ratio feedback control, moreover, an air/fuel ratio correction value is generally variably adjusted in response to that output turn. Therefore, the oxygen concentration detecting means has an output turning point (or first output turning point) at which the output is turned between the first output value and the second output value.
According to our experiments, when the emissions of the reduced components and the excess oxygen are augmented by the compression stroke injection in the vicinity of the stoichiometric air/fuel ratio, the output turning point of the oxygen concentration detecting means shifts from the stoichiometric air/fuel ratio in dependence upon the construction of the oxygen concentration detecting means. When an oxygen sensor used is prepared by coating the inner and outer surfaces of a cylindrical zirconia element with porous platinum electrodes having catalytic actions, for example, the sensor output turning point (or second output turning point) during the execution of the compression stroke injection in the vicinity of the stoichiometric air/fuel ratio shifts to the leaner side by an air/fuel ratio of about 0.3 to 1.0 from the first output turning point corresponding to the stoichiometric air/fuel ratio. By making the feedback control at the oxygen sensor output turning point shifted to the leaner side in the second mode, therefore, the reduced components and the excess oxygen can be optimized to efficiently raise the temperature of the catalyst disposed in an exhaust passage, for example.
In the aforementioned embodiment of the invention, preferably, the air/fuel ratio correction value to be made variable according to the output of the oxygen concentration detecting means in the air/fuel ratio feedback control is corrected to increase or decrease when the operation mode is switched.
According to the air/fuel ratio feedback control for making the air/fuel ratio correction value variable in response to the turn of the oxygen sensor output, the sensor output turning point is shifted to the leaner side, as has been described hereinbefore. Just after the operation mode switching between the first operation mode and the second operation mode, therefore, the shift of the output turning point of the oxygen sensor output is delayed to delay the optimization of the air/fuel ratio correction value (as should be referred to FIG. 8). Thus, a convergence into a new target air/fuel ratio accompanied with the operation mode switching may take a long time to deteriorate the stability in the air/fuel ratio control.
Therefore, the air/fuel ratio control is stabilized by correcting to increase/decrease the air/fuel ratio correction value at the operation mode switching time to forcibly bring the practical air/fuel ratio just after the operation mode switching close to a new target air/fuel ratio so that the time period required for the air/fuel ratio to converge into the new target air/fuel ratio may be shortened to stabilize the air/fuel ratio control. In other words, the air/fuel ratio feedback control, based on the new target air/fuel ratio, is started early, even when the fuel injection timing changes according to the switching between the suction stroke injection and the compression stroke injection.
In the aforementioned embodiment of the invention, more preferably, the air/fuel ratio correction value in the first operation mode is subtracted and corrected (refer to FIG. 6) at the time of switching the operation mode from the first operation mode to the second operation mode. When the second operation mode is switched to the first operation mode, on the other hand, the air/fuel ratio correction value in the second operation mode is added and corrected. This subtraction/addition correction may be a fixed value or may be determined according to the engine running state (e.g., the engine speed and the volumetric efficiency) from a preset map. According to this preferred mode of embodiment, the air/fuel ratio correction at the operation mode switching time is optimized.